Large structures, such as spherical gas holders storing municipal gas, bridge piers, plant pipelines, and blades of wind turbines, are subjected to periodic non-destructive inspection for deformations, such as cracks, to ensure their safety. However, a conventional non-destructive inspection, such as ultrasonic testing, is costly. For this reason, in many cases, costly non-destructive inspections are scheduled at longer intervals, with less costly visual inspections carried out between the non-destructive inspections.
However, visual inspections have certain shortcomings, such as the possible oversight of microscopic cracks and difficulty in working at heights.
To overcome these shortcomings, for example, an approach involving coloring cracked portions at the time of crack formation has been proposed to prevent the cracked portions from being overlooked during visual inspection. One approach involves paint with microcapsules containing color-developing solutions so that cracked portions are colored as a result of the microcapsules being ruptured at the time of crack formation. With this approach, however, it is still difficult to visually detect microscopic cracks because the extent of infiltration of the color-developing solutions into such microscopic cracked portions is small, resulting in an excessively small colored portion. In order to ensure a larger colored portion despite a small crack, there are at least two common approaches. One is to increase the number of microcapsules used and the other is to enhance the rupture sensitivity of the microcapsules. However, the former approach is not practical due to the inherent nature of the paint involved. The latter approach is not desirable because the microcapsules would be likely to also rupture even when a crack does not occur.
Therefore, there is a demand for a coating material capable of promoting visible reaction not only in a deformed portion but also in the surrounding area thereof when a surface of a structure deforms, so as to increase the size of a colored portion to a visible level, thereby making it easier to visually detect the deformation of the structure.